This invention relates to read and modify sequences of computer systems, and more particularly to modifying portions of data prior to all of such data being available from a main store.
A read, modify, write operation is a common sequence of events in computing systems which have a main storage interface several bytes wide. Prior art systems, such as the IBM System/370 series computers overlapped read and modify operations of data; however such modified data was required to be written back to the same main storage location it was read from. Many operations which do a read/modify/write, modify or replace only a fraction of the bytes which are read. The time the computer system spends waiting for the read to complete before modifying is significant.
U.S. Pat. No. 4,394,736 to Bernstein et al discloses a two level microcode system in which a first level of code has selected fields modified by a second level of code. U.S. Pat. No. 4,040,030 to Cassonnet shows an intermediate buffer for storing instructions. Transfer of the instruction for execution is prevented if a next instruction has the same address. In U.S. Pat. No. 4,084,623 to Gruner, a data processing system provides for an overlapping of the access operations such that access to a second memory module can be obtained by a processor unit before a data transfer has been completed with respect to a first memory module. The above patents do not discuss or provide insight to the problem of modifying data from main store before the data has been fetched.